SPIGOT-2540: Add nullability annotations to entire Bukkit API

By: Darkyenus <darkyenus@gmail.com>

paper-api/src/main/java/org/bukkit/entity/ZombieVillager.java

@@ -1,5 +1,7 @@
 package org.bukkit.entity;
 
+import org.jetbrains.annotations.Nullable;
+
 /**
  * Represents a {@link Zombie} which was once a {@link Villager}.
  */
@@ -9,7 +11,7 @@ public interface ZombieVillager extends Zombie {
      * Sets the villager profession of this zombie.
      */
     @Override
-    void setVillagerProfession(Villager.Profession profession);
+    void setVillagerProfession(@Nullable Villager.Profession profession);
 
     /**
      * Returns the villager profession of this zombie.
@@ -17,6 +19,7 @@ public interface ZombieVillager extends Zombie {
      * @return the profession or null
      */
     @Override
+    @Nullable
     Villager.Profession getVillagerProfession();
 
     /**